A closer look at herbicide rotation restrictions

Farm Forum

What herbicides were applied earlier in the growing season can significantly influence the decision of what to do in fields or areas of a field where the original crop was flooded or hailed out. Planting some kind of a crop can help reduce erosion potential as well as reduce the risk of fallow syndrome (see http://z.umn.edu/fallowsyndrome). However, options can be limited if a herbicide used previously in the growing season has rotational or plant-back restrictions listed on the label.

Replanting to a Crop that will be Harvested

If you want to plant a crop that will be harvested, or the crop will enter the food or feed chain, rotation or plant-back restrictions must be followed. Remember the label is the law. Severe crop injury potential is just one reason a rotation or plant-back restriction could be listed on an herbicide label. The restriction could also be in place to prevent the existence of illegal herbicide residue levels in the seed if the crop is harvested for grain, or in plant material if the crop is harvested for feed or forage. If the crop is to be grazed, grazing restrictions must be followed as well.

Planting Areas to a Cover Crop

The situation is a little different if you wish to plant a cover crop that is strictly planted for soil erosion protection or the prevention of fallow syndrome (i.e. the cover crop would not be harvested or grazed and it would not enter the food or feed chain). If the herbicide company doesn’t recommend or approve rotation to a particular cover crop, you would assume all risk for any germination or injury problems seen with the cover crop. But, there would not be the legal issues of trying to sell an “adulterated crop”, for example, as you are not harvesting the cover crop and it will not enter the food or feed chain.

Factors Affecting Herbicide Persistence and Degradation

When discussing rotation or plant-back restrictions, one may wonder how much of an herbicide actually remains in the soil in a flooded or hailed out field considering the high amounts of rainfall we’ve had recently. A quick review of what factors affect the persistence (amount of time an herbicide remains active in the soil) and degradation of an herbicide in the soil helps address this question.

Factors that affect the breakdown of an herbicide will influence the persistence of an herbicide in the soil. Factors that affect herbicide persistence can be divided into three categories: Soil, climatic, and herbicide chemical factors. These factors interact with each other as well.

Soil factors that affect herbicide persistence include soil texture, pH, organic matter, cation exchange capacity, and microbial activity. For example, persistence of imidazolinone herbicides increases as soil pH decreases while persistence of sulfonylurea herbicides increases as soil pH increases.

The dominant mechanism for herbicide degradation in the soil is through microbial activity. Conditions favorable for microbial populations, such as warm temperatures, oxygen, good fertility and a medium soil pH, will favor herbicide breakdown. In contrast, flooded soil conditions where oxygen is lacking, will reduce microbial activity and herbicide degradation.

Climatic factors involved in the degradation or breakdown of herbicides include moisture, temperature, and sunlight. Generally, herbicide degradation rates increase with increases in temperature and moisture, while cool, dry conditions slow degradation.

Finally, chemical properties of the herbicide affect the herbicide’s persistence, including the herbicide’s water solubility, soil adsorption, vapor pressure and susceptibility to chemical and microbial breakdown.

Herbicide Degradation Over Time

The measure of time used when discussing how long it takes a herbicide to degrade is called the herbicide’s half-life. A herbicide’s half-life is defined as the amount of time it takes for 50% of the chemical to degrade. The half-life of herbicides can vary greatly, ranging from a few days to a few years. Some herbicides may still be injurious to certain crop species after many half-lives, while other herbicides may persist longer but be less injurious to some crops.

Read the Label

The bottom line is to read and follow the label when looking at planting a crop or a cover crop into ground where a herbicide application has been made earlier in the growing season. Keep in mind that a delayed postemergence application this year could influence next year’s crop rotation. Also check herbicide labels for the maximum amount of active ingredient allowed to be applied in a growing season if you are faced with a replant situation.